In recent years, development of electronic devices in which GaN layers and AlGaN layers are sequentially formed on a substrate composed of for example sapphire, SiC, gallium nitride (GaN) or Si and in which the GaN layers are employed as electron travelling layers (compound semiconductor devices) has been active.
The bandgap of GaN is 3.4 eV, which is large compared with the bandgap of 1.1 eV of Si and the bandgap of 1.4 eV of GaAs. Consequently, operation at a high withstand voltage is anticipated in such compound semiconductor devices.
One example of such a compound semiconductor device is a GaN-based high-electron-mobility transistor (HEMT). Hereafter, a GaN-based high-electron-mobility transistor will be referred to as a GaN-HEMT. An HEMT is a field-effect transistor in which a high-mobility two-dimensional electron gas (2DEG) induced in a semiconductor heterojunction is employed as a channel.
When a GaN-HEMT is used as a power supply inverter switch, it is possible to achieve both a reduction in on resistance and an improvement in withstand voltage. In addition, compared with Si-based transistors, it is possible to achieve reduced power consumption during standby and improved operation frequency.
Consequently, it is possible to decrease the switching loss and the power consumption of the inverter. In addition, for transistors of equivalent performance, it is possible to make a GaN-HEMT smaller than a Si-based transistor.
However, a silicon MOS-FET of the related art is a normally off type (enhancement mode) of transistor that is off in a state in which a voltage is not being applied to its gate, whereas a GaN-HEMT is usually a normally on type (depression mode) of transistor which is on in a state in which a voltage is not being applied to its gate.
Consequently, there is a cascode transistor in which with an enhancement mode FET is used in combination with a depression mode GaN-HEMT so as to operate in an enhancement mode in order to switch the depression mode GaN-HEMT.
The following are reference documents:    [Document 1] Japanese Laid-open Patent Publication No. 2011-166673 and    [Document 2] US Patent No. 2012/0262220A1.